Locks having relatively large U-shaped brackets or shackles in combination with elongated housings have been used for preventing tampering and thwarting bicycle/motorcycle thieves. Early versions of U-shaped locks, such as the kind exemplified in U.S. Pat. Nos. 3,754,418, 3,924,426, and 3,967,475, were fabricated from flat pieces. These were susceptible to cutting by long-armed cutters. Later versions, such as those exemplified in U.S. Pat. Nos. 4,155,231, 4,545,224, and 4,918,949, were fabricated in a tubular shape for added resistance against cutting.
The U-shaped lock design nevertheless remains susceptible to tampering. One manner of tampering has been to cut the U-shaped bracket and wobble and twist the severed pieces of the bracket to chosen the ends of the bracket held in the housing. In each of the tubular designs cited in the patents above, at least one end of the U-shaped shackle is removably attached to the housing by either a detente or a bent hook end which, after the shackle is cut, can be wiggled within the housing and thus out of the gasp of the detente or hook. Consequently, the cut pieces of the shackle can be pried apart and the lock defeated.
Another way of defeating the lock is to spray the housing with freon to render the metal brittle, and then to shatter the metal of the housing and pry apart the ends of the U-shaped bracket. In many of the tubular lock designs in the patents mentioned, a lock or detente mechanism is located at the end of the housing there it is vulnerable to shattering and otherwise exposed to having leverage applied to pry the lock apart.
U.S. Pat. No. 4,584,855 teaches a U-shaped lock in which a cylinder lock is located within the housing between the openings which lockably receive the shackle ends. However, the lock require an access port to permit the key to be inserted into the lock, thereby disrupting the structural integrity of the housing. The disclosed detente cavities or gaps in the shackle end inserted into the housing, like locks of the above-mentioned prior art patents, do not substantially resist defeat of the lock by cutting and twisting. Furthermore, the lock requires numerous internal moving parts.
A lock having either a single or double U-shaped shackle is thus needed for defeating breakage by shattering and for resisting tampering by cutting and twisting.